The invention relates to the use of polycrystalline, porous alumina as a raw material for replenishing the melt which is consumed in growing monocrystals from the melt. Heretofore, the only satisfactory raw material has been what is known as crackle, a fully dense crushed boule of substantially single crystal alumina. The expense of using such a material has prompted the search for alternate materials. Sintered alumina, which costs substantially less to produce, is a likely candidate, however, direct substitution of the porous, sintered alumina for the crackle as a raw material has produced poor crystal growth and poor quality crystals. In particular, the light transmission of substantially monocrystalline tubing grown from the melt decreased by more than 50% with the raw material substitution. In addition to lower transmission of grown tubes, problems of actually completing the full growth of tubing were prevalent with the sintered material prior to the present invention. Refractory metal dies used in the growth process were also severely chemically attacked, limiting their use to one growth cycle.
Growth of crystalline or substantially monocrystalline bodies from a melt of material is not new. Various methods of growing crystals from the melt surface or from a die surface are known in the art. For example, see U.S. Pat. Nos. 3,471,266, 3,591,348, and 3,687,633, wherein a seed crystal is brought in contact with a temperature controlled melt or melt film and is subsequently pulled away from the melt as the crystallization of the melt takes place on the seed crystal. Various shaped crystals can be grown by appropriate use of refractory metal dies and control of thermal distributions and crystal pulling rates from the die surfaces.
The present invention is an improvement over these and other methods of growing crystals from the melt wherein the use of porous alumina raw batch for replenishing the melt is enabled.
The present invention is also an improvement over an earlier filed application Ser. No. 537,042, filed Dec. 27, 1974, now U.S. Pat. No. 3,960,503 and assigned to the present assignee. That invention concerned a novel feeder apparatus for use in crystal growing systems and as such may be used in the method of the present invention.